Fluvial–Estuarine Transitions in Fluvial-Dominated Successions: Examples from the Lower Pennsylvanian of the Central Appalachian Basin

Summary

Early Pennsylvanian sedimentation in the Central Appalachian Basin was dominated by the successive development of south- to southwest-flowing, low-sinuosity streams in broad, longitudinal braidplains, which deposited a series of quartzarenites. Successive quartzarenite belts are locally separated by grey shales with brachiopods and other body fossils interpreted to represent marine- to brackish-water facies. Local features indicative of tidal sedimentation occur between fluvial facies and the marine- to brackish-water shales. Tidal features occur in transgressive successions between fluvial and overlying marine to brackish-water shales, and significant wave-generated features are absent, indicating that tide-dominated estuaries developed during transgressions. The boundary between fluvial facies and recognizable estuarine tidal facies represents a fluvial–estuarine transition. Tidal sedimentary features in the fluvial–estuarine transition, however, can be subtle, because upper estuarine channels may record only the most headward tidal effects in an otherwise fluvially dominated system.

Some of the possible tidal indicators noted in upper estuarine channel facies include local occurrences of opposing palaeoflow indicators, non-cyclic rhythmites, lenticular bedding, small reversing ripples on the crests of underlying current ripples, sigmoidal cross-strata, cross-strata with rising troughs, thick–thin laminae pairs and bundled laminae in ripple cross-lamination. None of these features is diagnostic for tidal sedimentation. Where multiple tidal indicators are found within otherwise fluvial facies, within a probable transgressive succession, interpretation of an upper estuarine channel facies becomes more tenable.

Recognition of fluvial–estuarine transitions is important in fluvial-dominated basins, especially in the upper reaches of longitudinal basins, because the transitions may be the only evidence of correlative down-dip marine flooding surfaces. Identification of the transition zone facilitates the distinction between lowstand and transgressive systems tracts. In turn, such sequence analyses can increase the potential for predicating lateral changes in fluvial channel continuity, and vertical changes in porosity and permeability characteristic of lithological changes from fluvial to estuarine facies, both of which are important in exploration for hydrocarbons.